A growing number of businesses are building out on top of the Amazon platform. A couple of interesting discussions of this trend are noted here, one which draws comparisons to Google, the other with eBay. An article in MIT Technology Review:

While companies such as Google and Microsoft are also experimenting with the idea of letting outsiders tap into their databases and use their content in unpredictable ways (see "What's Next for Google?"), none is proceeding more aggressively than Amazon. The company has, in essence, outsourced much of its R&D, and a growing portion of its actual sales, to an army of thousands of software developers, who apparently enjoy nothing more than finding creative new ways to give Web surfers access to Amazon merchandise--and earning a few bucks in the process. The result: a syndicate of mini-Amazons operating at very little cost to Amazon itself and capturing customers who might otherwise have gone elsewhere. It's as if Starbucks were to recruit 50,000 of its most loyal caffeine addicts to strap urns of coffee to their backs each morning and, for a small commission, spend the day dispensing the elixir to their officemates.

By November 2004, the number of developers participating in Amazon Web Services had grown to 65,000. To keep up with their demands, the company has kept updating its APIs to open up more types of product information and more functions, such as wish lists and advanced searches. How many purchases originate each day with Amazon's growing web of syndicated storefronts? The company won't say, but experts have estimated that sites using the company's Web services send 10 million requests a day to Amazon's servers. [Welcome to Technology Review]

Web Services Pipeline compares eBay and Amazon:

Regardless of whether out-and-out convergence ever happens, a greater degree of interoperability may not be out of the question. There is a host of similarities between Amazon's and eBay's approaches: both depend on affiliates or merchant partners to make the business grow; both platforms require independent software developers to build merchant applications; both (according to Clark) are converting internal technology infrastructure to service-oriented architectures (SOAs); and both use Web services standards to open their platforms to outside developers. [Web Services Pipeline | eBay And Amazon: Dueling Technology Platforms Vie To Be The Center For Online Shopping]